1. Field of the Invention
The present invention relates to a method for reserving transmission capacities and for selecting requests for data streams of different bandwidth to be transmitted in digital data transmission networks having a maximum transmission rate, a particular transmission capacity being available which is less than or equal to the maximum transmission rate and, furthermore, certain bandwidths or bandwidth groups having particular data transmission rates in use. Certain data transmission rates ri are reserved for certain bandwidths or bandwidth groups bi, and a request for transmitting data having a particular bandwidth or bandwidth group bj only is accepted if predetermined criteria with respect to the utilization of the data network are met. In addition, the present invention relates to a data switching center.
2. Description of the Prior Art
Digital data transmission networks such as, for example, digital wire-connected telecommunication networks, mobile radio networks or ATM networks, pose the problem of appropriately dividing the total data transmission capacity over various services of different bandwidth. An example of this is the ISDN (Integrated Services Digital Network) which transmits voice data traffic in parallel at 1×64 kbit/s, pure data traffic at 128 kbit/s=2×64 kbit/s and video signals at 384 kbit/s=6×64 kbit/s. If a number of users are simultaneously dialing in for the individual services, there must be a decision criterion according to which the requirements for data transmission of the users are regulated and the total data transmission capacity is distributed over the services and within the services of different bandwidths.
It is obvious that the type of distribution of the data transmission capacity over the individual services is an essential criterion for the effectiveness and operational reliability of the data network. The aim of the distribution criterion is that the network:                is to operate effectively and with little blocking probability even at the edge of its design capacity;        is to respond flexibly to different load requirements of different bandwidths;        should be stable in the case of small deviations from its design capacity;        a large overload at one bandwidth should not trigger any blocking of other bandwidths;        both bandwidths of different magnitude should be treated with approximately equal chances (“fairness”); and        the administration of the data traffic should be simple.        
Different distribution criteria and algorithms can be used for largely meeting the abovementioned criteria.
For example, certain transmission channels can be unambiguously allocated to each bandwidth. This prevents any blocking of a bandwidth by another bandwidth. The disadvantage of this is that there is no flexibility for distributing the transmission capacity and, thus, an ineffective mode of operation results with changing requirements.
Another possibility lies in the entire transmission capacity being unrestrictedly available to all bandwidths up to the limit of capacity. Although this makes it possible to achieve optimum utilization, there is the possibility that the transmission of individual bandwidths is blocked by other bandwidths due to the high utilization of the data network. If a maximum capacity is reserved for each individual bandwidth in an improvement of this method, this leads to good protection against blocking between the bandwidths but to a low system efficiency.
Another possibility lies in basically providing the total transmission capacity to all bandwidths but to stop requests for transmitting data of a particular bandwidth as soon as the total available data transmission capacity drops below a certain threshold value. This method is called “sum limitation” or “trunk reservation” and is, in most cases, used together with priority allocations for certain requirements. The disadvantage of this method is that reservations are still maintained even at high overloads of certain bandwidths and, as a result, free capacity is wasted. This results in higher blocking probabilities for the other bandwidths in each case.
A method for reserving transmission capacities, and for selecting requirements for data streams of different bandwidth to be transmitted in digital data transmission networks having a maximum transmission rate, similar to the present invention, is known from European patent application EP 0 449 480 A3. In this document, it is proposed to accept a request for transmission of data having a certain bandwidth bj only if predetermined criteria with respect to the utilization of the data network are met. In this arrangement, various criteria are proposed, all of which relate to a dynamically varying total reserved data transmission capacity.
Furthermore, reference is made to the European Patent Application EP 0 798 942 A2 which discloses a method for reserving transmission capacities and for selecting requirements for data streams of different bandwidth to be transmitted in digital data transmission networks with a maximum transmission rate bmax, such that:                a particular transmission capacity is available which is smaller than or equal to the maximum transmission rate bmax;        certain bandwidths or bandwidth groups bi having data transmission rates si are in use;        certain data transmission rates ri are reserved for certain bandwidths or bandwidth groups bi,;        a request for transmitting data having a particular bandwidth or bandwidth group bj only is accepted if predefined criteria with respect to the utilization of the data network are met; and        a request for transmitting data having a certain bandwidth bj is accepted only if the unused data transmission capacity is greater than or equal to the sum of the reserved data transmission rates, with the exception of the reserved data transmission rate rj for the requesting bandwidth or bandwidth group bj even after the transmission has been accepted.        
It is, therefore, an object of the present invention to specify another method for reserving transmission capacities and for selecting requests for data streams of different bandwidths to be transmitted in digital data transmission networks having a maximum transmission rate with an improved and simplified criterion for the acceptance or rejection of a request for data transmission. It is a further object of the present invention to specify a data switching center which has an improved algorithm for accepting or rejecting a request for data transmission.